Flux-cored arc welding (FCAW) is a welding process where the heat is generated by an arc between a tubular wire electrode continuously fed into a welding machine and work piece. The wire electrodes used for the FCAW process comprise a metal sheath surrounding a core of fluxing and/or alloying elements and compounds. Typically the arc and the molten metal are shielded from the surrounding environment by a shielding gas. The chemical composition of the sheath, the fluxed core and the shielding gas determine the composition and properties of the resulting weld. The consumable electrode melts in the arc and is carried across the arc in a stream to become the deposited filler metal. Shielding of the arc is provided either by the flux contained in the tubular wire electrode or by an externally provided shielding gas.
The FCAW process is accomplished by a welding machine which is operated by a power source. The welding machine feeds a consumable electrode and provides the shielding gas into the welding process. The welding machine is powered by a power source which provides the electric power of the voltage and amperage necessary to maintain the arc. Most welding machines for the FCAW process operate on 110, 230 or 460 volt input power. The power sources used for the FCAW process are usually direct current constant voltage sources, but often use higher currents than gas-metal arc welding process, requiring a larger power source.
FCAW is a direct current welding process. The consumable electrode wires for FCAW designed to work with external gas shielding are normally used in the direct current electron positive welding configuration (DCEP). DCEN is typically used for self-shielded arc welding.
Since in the FCAW process a consumable wire electrode must be fed into the welding machine, a wire feeding system is required to provide continuous feeding. Several wire feeding systems are available and can be used in the FCAW process. Most of the systems provide a constant feeding speed and are used with a constant voltage power source. If a variable speed wire feeding system is used, a voltage sensing circuit is provided to maintain the desired length of the arc by varying the feeding speed of the wire. A wire feeding system usually comprises an electrical rotor connected to a gear box with a number of driving rolls.
In externally shielded FCAW welding machines, a shielding gas system supplies a shielding gas from a gas source (liquid in storage tanks or compressed in gas cylinders) to the working space to shield the arc from the surrounding environment. Typically, a shielding gas system comprises a source, a gas supply hose, a gas regulator, control valves and a hose supplying the gas to the welding machine. Shielding gases, which can be inert and/or active gases, surround the arc and a puddle of molten metal. The most commonly used shielding gases in the FCAW process are Ar, CO2, Ar—O2 mixture, Ar—CO2 mixture. The choice of a particular shielding gas or mixture depends on the type of metal to be welded, arc and metal transfer parameters, properties of the weld and bead shape.
Only very few filler metals are known to be suitable for welding structural steels which have a yield strength of 100 ksi and higher, requiring good impact toughness at low temperatures. The problem with the filler metals is known to be poor weld metal toughness, hydrogen cracking and hot cracking of the final weld metal. The cold and hot cracking problems draw particular attention in structural welding, such as support structures for pipelines or oil rigs, where it is undesirable to have any cracks in the weld metal joining two metal pieces. Stick electrodes used in the stick-metal arc welding process (SMAW) which can provide the desired mechanical properties in structural steels, but the SMAW process is much slower, and therefore less productive, than the FCAW process (the fluxed core heats up and melts faster, transferring and depositing the filler metal on the work piece faster). For this reason FCAW is frequently used for welding ferrous metals, such as steels, when high deposition rates are desired.
Problems such as cold and hot cracking have been particularly undesirable in weld joints in structural steels. One of the reasons causing cold cracking is a relatively high amount of hydrogen or water in a consumable wire electrode, causing the resulting weld metal to contain quantities of hydrogen sufficient to cause cold cracking. Molecules of water present in the fluxed core dissociate in the welding arc into hydrogen and oxygen. Some amount of dissociated hydrogen and oxygen will diffuse into the molten weld pool during the welding process. As the metal cools, hydrogen trapped inside diffuses and concentrates on the defects inside the metal. If the concentration of hydrogen on the structural defects and the residual stresses caused by the welding process are sufficiently high, cracks will form in the weld metal.
During welding the consumable wire electrode enters the arc, melts and gets transferred to the work piece to form a molten weld pool and a molten slag pool. The slag pool solidifies first, and the molten metal solidifies later, taking on the shape of the solidified slag. As the metal solidifies, dendrite crystals form and grow in the direction of solidification, which is the same as the direction of welding. The formation and growth of the dendrite crystals results in creating of the area of different alloying concentrations, known as “partitioning”. For alloying elements with a small atomic radius, such as, for example, Boron, their high diffusion rate allows them to diffuse in high enough concentrations to the inter-dendritic areas of the weld, causing differences in the melting points of the inter-dendritic areas and the parent weld metal. The areas with lower melting temperatures will not able to withstand the stresses caused by solidification, and the hot cracks will form. The higher concentrations of Boron make hot cracking more likely to occur. If a crack propagates through a weld, a structure joined by such a weld may not conform to the desired strength specifications.
To obtain the desired mechanical properties of the welded joints when welding structural steels, the alloying systems providing those mechanical properties, but containing only limited amounts of the elements lowering the temperature of the start and the end of the martensite transformation in the welded joints, have been used. Formation of hard, martensitic heat-affected zones caused by higher cooling rates is undesirable, because martensitic structure bears a higher risk of cracking during cooling. Such alloying elements can be C, Mn, Cr and Ni. For example, a submerged arc welding process (SAW) with no shielding gas described in “Effects of Cobalt On The Structure and Properties of High Strength Weld Metal”, Avt. Svarka 1984, No. 7, pp.45-48. Welding high strength base metal is accomplished by using submerged arc welding which utilizes a fluxed core welding wire or a solid wire and a flux material not contained in the wire, but provided externally. The flux material provides shielding of the weld metal from the atmosphere by melting and forming a slag over the pool of molten weld metal during welding. The described SAW process is limited to flat and horizontal welding positions.
Therefore, reducing cold and hot cracking in weld joints of steels welded by a welding process providing high deposition rates would be highly desirable. Since there is a tendency in many industries to use high strength steel in order to reduce the amount of steel needed to complete a project, reducing cold and hot cracking in higher strength steels, such as 100 ksi and higher, is especially desirable. Therefore, the demand for the high strength low alloy filler metal is expected to rise.